An antenna apparatus for vehicle-mounted radar defines a predetermined angular region as a detection area, transmits radio waves within the detection area, and receives the waves reflected from object(s) present in the detection area. Typically, it is difficult to design an antenna that transmits radio waves only within the detection area, and some of the radio waves leak outside the detection area. The leaked radio waves, which are unnecessary waves, may be reflected from the outside of the detection area, and these reflected waves are known to deteriorate antenna characteristics in some cases. PTL 1, for example, discloses a technique for preventing this problem. The disclosed technique involves providing a radio waves absorbing layer, which minimizes leakage of radio waves outside a detection area, on side surfaces of a cover member formed of material through which radio waves can pass through, so as to protect antenna elements that transmit radio waves.